In recent years, many studies have been made on use of useful substances converted from recyclable biomass resources produced from plants and the like. Cellulose contained in a plant biomass as a major component is characterized by being insoluble in water or a usual solvent and being persistent because it is a polymer formed of β-1,4-linked glucose units, forms hydrogen bonds within and between molecules, and thus exhibits high crystallinity. In recent years, a study on a reaction using a solid catalyst that is recyclable and can reduce an environmental burden has been made as a cellulose hydrolysis method which substitutes for a sulfuric acid method or an enzyme method.
The hydrolysis reaction of cellulose with a solid catalyst is a solid-solid reaction, and a rate of the reaction is limited by contact property of the catalyst and a substrate. Therefore, in order to realize a highly-efficient reaction, it is necessary to develop a highly active catalyst as well as a treatment method of improving reactivity. For example, as a method of improving reactivity in a solid-solid reaction system, there are given a method involving mixing and preheating a pulverized substrate, a catalyst and preheating steam (JP-A-2008-297229, Patent Document 1) and a method involving allowing a catalyst and a substrate to react under irradiation with microwaves (JP-A-2010-98994, Patent Document 2).
However, Patent Document 1 discloses that about 70% of cellulose is degraded, but does not specifically describe the yield of a sugar obtained as a degraded product, and the effect is unknown. In addition, in Patent Document 2, the glucose yield is about 30%, and a high reaction yield has not been achieved. Further, it is necessary to introduce an expensive microwave irradiation apparatus, and the method is problematic in practicality.
In addition, as a method of improving reactivity in a pseudo-liquid-solid reaction system, there is given a method involving adding cellulose to a cluster acid catalyst in a pseudo-molten state to perform hydrolysis (JP-A-2008-271787, Patent Document 3). However, the method of Patent Document 3 is problematic in practicality due to difficulty in controlling water content during a reaction, requirement of many steps for separation of the catalyst from the product, and use of an organic solvent.
For the above-mentioned reasons, it has been desired to establish a pre-treatment method for a hydrolysis reaction raw material that can achieve a high sugar yield without introduction of an expensive apparatus and addition of a further component in a hydrolysis reaction of a plant biomass with a solid catalyst.